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VC++ Example: RegisterWindowMessage,
WM_USER, ON_NOTIFY

Shaman Pintzky.

When sending
messages and notification from user-created
controls, one has two options to define custom messages.
The first is to send a WM_USER
message. That is, take the WM_USER
constant and add to it a number. For example:

#define SPECIAL_CTRL_CHANGE(WM_USER + 1000)

This allows any window to
capture the message or notification using normal ON_COMMAND/ON_NOTIFY
macros. However, using WM_USER
for normal messages is a mistake. Suppose I created a
component called "Special" and another
programmer from another side of the galaxy creates a
component called "VerySpecial". Both of us could
have several messages sent and both would use WM_USER
messages. If by any chance both controls will use the same
message, a windows program might behave in an unexpected
way, since both use the exact same message. This problem
exists only when using the ON_COMMAND
macro and not ON_NOTIFY,
since ON_NOTIFY uses
the control id and not just the message code.

Therefore the conclusion
is to use WM_USER
messages only when sending notification messages and not
normal windows messages.

One alternative to using
custom Windows messages that are based on the WM_USER
message is to register your own Windows messages. The idea
behind registered messages is to create a unique Windows
message based upon a string. Any window that knows the
string can obtain the message code and therefore, respond
to the predefined message. This allows several components
to create custom messages, and by simply concatenating the
component name to the message name, they can be sure that
their string is unique, and therefore the message code is
unique.

Here's an example of how
to add the message handler of a registered
message on the client side using the MFC
message map.

Another nice advantage that
registered message can provide, is the ability to capture
the message sent from another application, again by only
knowing the string.

The Windows Common
Controls use this type of message routing from the DLL to
the user application for several messages such as
FindOrReplace command in RichEdit control. The
intellimouse messages are also sent with registered
messages, instead of creating new windows messages for
them.

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